Multi-User MIMO (MU-MIMO) is an effective technique to increase system throughput in a wireless communications system. In general, a communications system using MU-MIMO allows the simultaneous transmission of information to multiple users from a single base station over the same frequency band. This may result in increased overall data rate and reduced user latency.
A problem often associated with implementing MU-MIMO effectively is that downlink (DL) control overhead can quickly increase with the number of users since each user needs to have information about all the other simultaneously scheduled users. The information about the other simultaneously scheduled users may be used for interference cancellation. In both a Long Term Evolution (LTE) and Universal Mobile Broadband (UMB) cellular communications systems with MU-MIMO, two solutions have been proposed: (1) limit the number of scheduled users to two and indicate the interfering vector, or (2) use a dedicated pilot. With either approach, the number of combinations between simultaneously scheduled users and their reported channel quantization grows exponentially with the number of users.
In general, there are two forms of MU-MIMO in an LTE communications network: unitary matrix based precoding and non unitary based precoding. In unitary matrix based precoding, a user (or mobile station (MS)) indicates a preferred precoder vector belonging to a known matrix. A base station (BS) then groups together multiple MSs reporting precoder vectors belonging to the same matrix. An advantage of unitary matrix based precoding is that DL control can be performed efficiently. However, MSs are grouped based on reported precoder vectors of the same matrix, which means the codebook is correspondingly small. Therefore, channels cannot be well aligned with the codebook.
In non unitary based precoding, such as zero-forcing beamforming, for example, a MS reports its channel to the BS (typically, a quantized version of the channel) using a known codebook. The BS then collects channels from all MSs and constructs a precoding matrix that is a function of the channels of the MSs and transmits the precoding matrix to the MSs. An advantage of non unitary based precoding is that the codebook may be large, therefore, channels can be well aligned with the codebook.
Therefore, there is a need for techniques that do not exhibit exponential growth in DL control structure overhead with increased MSs.